The present invention is related to processes which utilize heat regenerable liquids for the removal of sulfur dioxide from gas streams and to the production of elemental sulfur from sulfur dioxide extracted from gas streams.
Many processes employ aqueous solutions of ammonia, alkaline salts of organic and inorganic acids, and aromatic amines as absorbents for the extraction of sulfur dioxide from gas streams. These solutions are readily stripped of sulfur dioxide upon the application of heat and therefore regenerated for reuse. The stripped sulfur dioxide is processed for the production of liquid sulfur dioxide, sulfuric acid or elemental sulfur.
To produce elemental sulfur from the recovered sulfur dioxide, it is expedient to react the sulfur dioxide with hydrogen sulfide by the Claus reaction. This requires two moles of hydrogen sulfide for each mole of sulfur dioxide.
In a situation where a source of hydrogen sulfide is not available as, for example, in a power plant where sulfur dioxide must be removed from stack exhaust gases, hydrogen sulfide has to be generated if sulfur is to be produced.
This may be accomplished by converting two thirds of the stripped sulfur dioxide to hydrogen sulfide and reacting the balance with the formed hydrogen sulfide over an alumina catalyst in the catalytic zone of a typical Claus plant.
Another possibility, which has been proposed for the regeneration step of the Citrate process, is to produce hydrogen sulfide by reduction of elemental sulfur generated in the process for reaction with sulfur dioxide dissolved in the citrate solution.
Known methods of producing hydrogen sulfide from elemental sulfur have been described, for instance, in "Canadian Mining and Metallurgical Bulletin", Oct. 1957, p. 614 and following, and "Mining Engineering", Jan. 1970, p. 75 and following.
The former process involves non-catalytic direct reaction of hydrogen with sulfur to form hydrogen sulfide at temperatures from 820.degree. to 1000.degree.F. An admitted deficiency in the process is that the reaction products, i.e., a mixture of unreacted sulfur and hydrogen sulfide, are highly corrosive. Type 316 stainless steel, for instance, suffers severe corrosion. In addition, hydrogen of high purity is required for the process and this is expensive.
The process described in the latter publication involves two conversion stages. In the first, sulfur is reacted with a hydrocarbon, such as methane, at a temperature from 600.degree. to 700.degree.C. over a catalyst to form a mixture of hydrogen sulfide and carbon disulfide. The gas stream is passed to a second stage where, at a temperature of 200.degree. to 300.degree.C, the carbon disulfide reacts with water to form hydrogen sulfide and carbon dioxide. This process also suffers from severe corrosion problems and the net gas stream can contain considerable quantities of carbon-sulfur compounds such as carbonyl sulfide and carbon disulfide.
As it pertains to the operation of the Citrate process, it would be necessary to convert two thirds of the formed sulfur to hydrogen sulfide for reaction with sulfur dioxide in the liquid phase at ambient temperature. This is an expensive operation from an energy conservation standpoint since two-thirds of the product must always be recycled back in the form of hydrogen sulfide. Further, the proposed means to generate hydrogen sulfide from the formed sulfur leaves much to be desired due to the corrosion and pollution problems attendant to the generation of hydrogen sulfide from the elemental sulfur.